In digital radiography X-ray opaque material is used to protect subjects against unnecessary exposure to X-rays, to limit the radiation scattering and to obtain multiple irradiation fields on a recording medium such as a stimulable phosphor sheet. The region outside the irradiation fields will have high luminance when the image is displayed on a display device. The strong light will have a negative impact on the efficiency and accuracy of the diagnosis.
Therefore automatic recognition of the irradiation field and blackening of the region outside the delineated irradiation fields is an important part of image processing of digital radiographic images.
Prior art techniques are edge based. The digital greyscale image is searched for pixels constituting the boundary between irradiation fields and the shadow of the x-ray opaque material. These pixels are grouped into candidate line segments to obtain a correct delineation of the irradiation fields. The candidate line segments are evaluated against a rule set of often local image characteristics. A patent following this general idea for irradiation field recognition is EP 0610605. Another patent based on this idea is U.S. Pat. No. 5,901,240.
These edge-based approaches may fail in cases where the edges are hardly distinguishable in the image. This is the case for images with high scatter radiation. In these images the transition between irradiation field and the shadow of the x-ray opaque material is not an edge but a gradual transition zone.
Another category of images that may cause failure of the edge-based approach are low dose images with a very low contrast between the irradiation field and the shadow of the x-ray opaque material. An example is a lateral nose image in which the transition between the dense bone of the skull and the shadow of the x-ray opaque material has low contrast.
It is an aspect of the present invention to provide a method for extracting the irradiation fields in an x-ray image that overcomes the above disadvantages.